The present invention relates generally to voice switching in loudspeaking telephone systems and is more particularly directed to ambient noise supression in a bistable voice control with hysteresis.
A loudspeaking telephone or one that may be operated "hands free" is generally known in the art to include a microphone and transmitting channel connected to the input of a hybrid line transformer that is coupled to a telephone line, and a speaker and receiving channel connected to an output of the hybrid.
Generally the transmitting channel and receiving channel also contain means to provide gain for the signals flowing from the microphone or to the speaker. This gain in the receiving and transmitting signal amplitude and power may be accomplished in many ways with different types of electronic amplifiers including tubes, transistors, integrated circuits, and large scale integration.
One of the more common configurations is to provide the transmitting channel with a microphone pre-amplifier having high voltage gain and to then use its output to power a line driving amplifier having high current gain attached to the hybrid. Likewise, the receiving channel has a receive amplifier having a high voltage gain receiving an input from the hybrid and driving a speaker amplifier having enough current gain to ensure sufficient current to a speaker. There may be further matching or intermediate amplifiers or the transmit or receive amplifiers may have more than a single stage in either of the channels depending upon design.
This configuration, however, is unstable as the hybrid line transformer does not constitute a perfect balanced match to the telephone line. This mismatch causes leakage of part of the transmission signal into the receiving channel and its subsequent appearance as acoustic at the speaker. The microphone will then cause a regeneration of the signal through the system to establish a positive feedback loop and consequent instability.
The phenomenon is generally termed "singing" and occurs when the loop gain is greater than unity. The gain of the loop is equal to the gain of the receiving channel and the transmitting channel less hybrid loss and acoustic loss between the speaker and microphone. Thus, advantageously the circuitry should be designed to make the sum of the gains less than the sum of the losses around the loop.
The prior art generally attempts to solve this problem by "voice switching". The "voice switching" technique usually provides a control element which is bi-modal, being either in a transmission state or in a receiving state. While in one state the control will attenuate the gain of the channel not being used so the loop gain will be less than unity.
Attenuation of the channel gain in a loudspeaking telephone is generally accomplished by increasing the impedance or loss in a series element in the channel or decreasing the gain of one or more of the amplifiers in the channel. When the control switches states, the channel not being used is restored to full gain while the other channel's gain is decreased. Essentially the operation becomes bistable with one state being a receiving mode with the transmitting channel muted and the other being a transmission mode with the receiving channel muted.
The control element in "voice switching" has its state changed from transmission mode to receiving mode and vice versa by signals provided from the channels indicating the direction of seizure needed. Acoustic energy which is transduced by the microphone into an electrical signal is transmitted to the control which in turn indicates the desire to seize the control of the transmission channel, while an input from the hybrid results in the transmission of an electrical signal to the control which indicates the desire to seize the control for the receiving channel.
Although "voice switching" substantially solves the problem of singing in loudspeaking telephones, a number of other problems associated with its operation are of concern.
One such problem is echo which may be picked up by the microphone from the sound produced by the speaker in a reverberant environment and re-transmitted to the connected station over the telephone line. This phenomenon occurs when the inherent mechanical delay caused by the energy being reflected from a surface is of sufficient magnitude to cause the tripping of the control into its transmitting mode.
Echo may cause at least two undesirable results at the transmitting station. One result is that of a re-echo which again causes re-transmission with the possibility of causing instability within the system. Another undesirable result is the disconcerting effect echo has upon the individual speaking into the telephone. It is known in the prior art that the longer the delay the more disconcerting the echo becomes to that individual.
In addition to the echo problem in voice switching, premature seizure of a mode during syllabic pauses in the received speech may occur when there are simultaneous vocalizations (double talking) by both parties to a conversation or when ambient noise causes a transmit seizure at the receiving station during a pause in the conversation.
In the event of premature seizure, a rapid switching of the control from state to state and a clipping of each syllable transmitted often takes place. The clipping is caused by the time delay required by the control to transmit each new syllable after a change of state. The transmitting station will receive flutter static and the received speech will sound choppy because of this clipping.
Premature seizure may be eliminated by preventing the bistable control from switching at the same transmit and receive levels by providing different switching levels for switching into the transmit and receive modes. The difference between the switching levels is termed hysteresis and in operation results in an overtake of a predetermined minimum magnitude which the bistable control must recognize before it changes states.
The prior art discloses the use of an artificial gain parameter generated by an amplifier to produce hysteresis to bistably switch the gain of the amplifiers. Such prior art apparatus is susceptible to feedback instability problems as it connects the output of an amplifier to its input.
Also, the hysteresis technique developed in the prior art may not be utilized in systems that use variable impedances instead of variable gain amplifiers for the switchable gain degraders in each channel because such impedances cannot generate an artificial gain "parameter".
Another problem in prior art voice controlled loudspeaking telephones with hysteresis has been poor overtake performance. Mainly the problem is caused by ambient noise conflicting with the receiver channel signal for seizure of the bistable control. The noise may hold the circuitry in the transmission mode even though transmission has been completed by one party and the other party begins talking.
It would be advantageous to eliminate the noise parameter from the transmitted signal and to eliminate any other spurious parameter from either of the transmit and receive signals that is generated by the transmit signal to prevent such poor overtake performance. One such parameter is the signal generated by the hybrid (due to the leakage described above) which is combined with the received signal and if transmitted to the control circuitry further effects system performance by causing "breakthrough" blocking.
The reception of faint signals -- which, for example, may be encountered on lines that are connected to a plurality of first stations that are relatively close to each other and to a plurality of second stations that are relatively distant, from the first stations -- has been still another problem in voice switched loudspeaking telephones. If the receiving channel has sufficient gain to amplify faint signals to a reasonable power level and a control sensitive enough to respond to the amplified signals, then strong signals will be amplified to a level which is uncomfortably loud and any hysteresis provided in the system will be ineffective.
The prior art has attempted to overcome this problem by the introduction of limitors and various types of automatic gain control ("AGC") circuits which may cause further problems such as "AGC bursting". What is actually needed is a simplified way of increasing the receiver gain for substantially all faint signals below a certain signal level so that they may produce a reasonable audio level at the speaker without detrimentally affecting the voice control sensitivity.